Light is known to interact with endogenous or exogenous chemical agents in the skin or eyes, to produce photosensitization (phytotoxicity or photoallergy). The objective of this study is to determine whether light-induced free radicals play a role in photosensitization. Electron spin resonance studies, in conjunction with spin trapping techniques, have shown that most halogenated aromatic photosensitizers, e.g. amiodarone, bithionol, fentichlor, chlorpromazine, undergo dehalogenation upon UV irradiation to yield the corresponding aryl radicals and halogen atoms. These aryl radicals were capable of abstracting hydrogen atoms from suitable donors, suggesting that in vivo they could initiate peroxidation by reacting with unsaturated lipids. UV- irradiation of bithionol, fentichlor and related 4-chlorophenols also yielded the corresponding semiquinone radicals which may interact with the mitochondrial electron transport chain. Semiquinone radicals also disproportionate to give the parent quinones which are known to react covalently with nucleophiles (NH2, SH). Irradiation of hairless mouse (HR/J) skin containing bithionol resulted in the generation of free radical intermediates, which may be involved in the in vivo photo-sensitization reaction to this compound. Other photosensitizers which also generate free radicals upon UV-irradiation include anthracyclines, tetracyclines, anthralin, and the salicylanilides. In order to facilitate the full use of available spin-trapping information in the literature, a database has been designed and implemented on an IBM PC/AT. The database presently contains approximately 600 references spanning a period of two decades (1967-1987).